Technical Field
The present invention relates to an attenuated vaccine strain against mycoplasmal pneumonia of swine (MPS), a live vaccine containing the attenuated strain, and use thereof in the protection against Mycoplasma hyopneumoniae (Mhp) infection related diseases.
Related Art
Mycoplasmal pneumonia of swine (MPS), common name pig asthma, is an infectious chronic respiratory disease of swine caused by Mhp, and the symptoms mainly include cough and shortness of breath. The disease mainly leads to decreased feed conversion rate and retarded growth of swine, and is characterized by high incidence and low mortality. MPS is highly prevalent worldwide, and brings about heavy economic losses to the modern swine industry.
Mhp generally forms, together with other pathogens, mixed infections. For example, swine enzootic pneumonia is induced by mixed infections with Pasteurella Multocida, Streptococcus Suis (SS), Haemophilus Parasuis of Swine (HPS), or Actinobacillus Pleuropneumonia of Porcine (APP). When mixed infections are caused by Mhp and Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), Porcine Circovirus Type 2 (PCV2) and/or Swine Influenza Virus (SIV), common Porcine Respiratory Disease Complex (PRDC) takes places, which in turn causes not only respiratory system diseases, but also a decreased reproductive ability of the swine.
MPS is a refractory chronic infectious disease for which the therapeutic effect is less desirable, and that may easily reoccur after drug withdrawal and is difficult to be eradicated once it is epidemic in the pig farm. Therefore, the prevention, control and elimination of MPS are accomplished by means of comprehensive measures such as early diagnosis, timely isolation, antibiotic treatment or removal of affected pigs, vaccine prevention, all in and all out, and strict disinfection, where vaccine prevention plays a critical role in the control of MPS.
Although inactivated vaccines for MPS are currently massively used in clinic, the control effect for this disease is unsatisfactory. Moreover, the inactivated vaccines generally can lower the disease index of the lung of the affected pigs to some extent, but cannot decrease the proportion of diseased pigs effectively. MPS is accordingly considered as one of the infectious diseases of swine that are most widely epidemic, and most difficult to be eliminated. The poor effect of the vaccines may be attributed to the nature of the inactivated vaccines and the characteristics of Mhp infection.
Mhp binds to and is colonized on the bronchial ciliated epithelial cells through respiratory infection, thus causing persistent infection. After immunization with inactivated vaccines, systemic immune response is elicited; however, the local immune response inducing effect is poor. The circulating antibody produced needs to penetrate through the epithelium barrier, and be secreted into the tracheobronchial lumen. The process is quite difficult and suffers from low efficiency. For the purpose of effective immunization, the vaccine is required to stimulate the immune system to respond at a high level. Definitely, this necessitates relatively high dose of antigen and potent adjuvant, which lead to increased cost of the vaccine. Meanwhile, it is difficult to achieve the desirable control for the disease, because the immune response elicited by the inactivated vaccine can only inhibit the proliferation after infection with field strain and alleviate the tissue destruction, and cannot prevent the infection with field strain totally.